Compact two-stroke internal combustion engines have been used as a power source of portable power working machines such as brush cutters, chain saws or the like. Two-stroke internal combustion engines used in portable power working machines are driven at a very high revolution speed as much as near 10,000 rpm or ten plus some thousands rpm in normal revolution.
Compact two-stroke internal combustion engines generally have cylinder-port designs. In these two-stroke engines having such a cylinder-port design, air-fuel mixture ports and exhaust ports are formed in a sidewall of a cylinder, and opened or closed by a sidewall of a reciprocating piston. That is, cylinder port type two-stroke engines have no valve mechanisms dedicated to controlling intake and exhaust functions. Without complex valve mechanisms, cylinder port type two-stroke are made up of a reduced number of parts and so much reduced in weight. Therefore, they are suitable for use as compact power sources of portable power working machines.
Two-stroke internal combustion engines used as a power source in portable power working machines have crankcase compression designs. In engines having a crankcase compression design, air-fuel mixture is introduced into a closed internal space of a crankcase, i.e. a crank chamber, and the air-fuel mixture is pre-compressed by a descending movement of the piston. More specifically, when the crankcase is sealed and the piston moves up, the crank chamber is reduced in pressure to a vacuum. Using this vacuum, air-fuel mixture is next introduced into the crank chamber. Thereafter, in an expansion stroke where the piston moves down, the air-fuel mixture now existing in the crank chamber is pre-compressed by the descending piston, and it is injected into the combustion chamber at near the bottom dead center. The air-fuel mixture injected into the combustion chamber is utilized as a scavenging flow to force out any exhaust gas existing in the combustion chamber through the exhaust ports.
Because two-stroke internal combustion engines use fresh air-fuel mixture to scavenge the combustion chamber, they involve the problem of the so-called “blow-by” in which a part of fresh air-fuel mixture, not having burnt, is forced out together with the burnt gas.
Since the “blow-by” of air-fuel mixture is the phenomenon of undesired external discharge of fresh air-fuel mixture, it not only decreases the fuel efficiency but also increases harmful elements (such as HC, CO, etc.) in the exhaust gas discharged through the exhaust ports.
Stratified scavenging has been proposed as one of techniques for alleviating the “blow-by” of air-fuel mixture (Patent Documents 1, 2 and 3). The “stratified scavenging” is called “initial scavenging by air” as well.
With reference to FIG. 1, the intake system of a stratified-scavenging two-stroke internal combustion engine 1 includes an air-fuel mixture passage (MIX passage) 2, through which air-fuel mixture generated by a carburetor flows, and an air passage (AIR passage), through which fuel-free air containing no fuel flows. The air-fuel mixture passage 2 is in communication with the crank chamber 5 via an air-fuel mixture port 4. The air passage 3 is in communication with a cylinder wall air opening 6 formed in the cylinder wall, in case of an in-piston passage design, for example. This air passage 3 further communicates with a scavenging channel 8 via an in-piston passage 7. The scavenging channel 8 communicates with the combustion chamber 10 through a scavenging window 9 covered and uncovered by the sidewall of the piston P, and communicates with the crank chamber 5 through an air-fuel mixture feed port 11. Reference numeral 12 in FIG. 1 denotes an ignition plug.
In the stratified-scavenging engine, fuel-free air is introduced into the scavenging channel 8 through the air passage 3 by making use of the vacuum created in the crank chamber 5 by upward movement of the piston P. Subsequently, just after the exhaust port 12 is uncovered by downward movement of the piston P, the scavenging window 9 opens, and scavenging takes place. In the scavenging stroke, fuel-fee air in the scavenging channel 8 is first forced out into the combustion chamber 10 through the scavenging window 9 under a pressure in the crank chamber 5, and air-fuel mixture pre-compressed in the crank chamber 5 is next forced out. Thus, the combustion chamber 10 is stratified-scavenged.
Two-stroke internal combustion engines involve the problem of “blow-back” in addition to the problem of “blow-by”. The blow-back occurs even when the air-fuel mixture port 4 has a reed valve,
Engines of crankcase compression designs introduce a fresh air-fuel charge into the crank chamber 5 by displacement of the piston P as explained above, and the fresh charge is pre-compressed in the crank chamber 5. That is, induction of a fresh charge of air-fuel mixture into the crank chamber 5 takes place in the upstroke of the piston P, and the charge is pre-compressed in the crank chamber 5 in the downstroke of the piston P.
In piston valve designs, i.e. piston-controlled designs, when the piston P moves to near the top dead center, the air-fuel mixture port 4 opens and allows the crank chamber 5 to communicate with the air-fuel mixture passage 2 to introduce a charge of air-fuel mixture into the crank chamber 5. In this configuration, the air-fuel mixture enters into the crank chamber 5 from the air-fuel mixture passage 2 in the upstroke of the piston P to the top dead center. In the next expansion stroke, however, in which the piston P moves down, an increase of pressure in the crank chamber 5 due to the downward movement of the piston 5 causes blow-back of the air-fuel mixture from the crank chamber 5 to the air-fuel mixture passage 2. In reed valve designs using a reed valve (not shown) in the air-fuel mixture port 4, the reed valve exhibits a shutting behavior as the pressure in the crank chamber 5 rises in the process of descending movement of the piston P from the top dead center, and the shutting behavior of the reed valve invites blow-back of the air-fuel mixture to the air-fuel mixture passage 2. Blow-back of air-fuel mixture becomes more notable as the engine revolution increases, and here occurs the problem that fuel components and oil components in the air-fuel mixture, which flows back down to an air cleaner, pollute the air cleaner elements.
Japanese Patent Laid-open Publications Nos. 2000-170611 and 2006-144798 deal with the problem that a flow of air-fuel mixture having entered into the air cleaner by blow-back intrudes into the air passage, and partition the interior space of the air cleaner into two chambers such that the air-fuel mixture passage and the air passage open to different ones of these chambers.    Patent Document 1: U.S. Pat. No. 6,857,402    Patent Document 2: Japanese Patent Laid-open Publication No. 2000-170611    Patent Document 3: Japanese Patent Laid-open Publication No. 2006-144798